Fetch event loop from Timer object in Process tests
Prefer missing exit code to exception for missing sigchild compatibility

This removes a possible thrown exception from getExitCode(), which might have only shown up when the "exit" event was fired. Instead, do not allow sigchild compatibility to be changed once a process is running.
Test Process::start() exception if process is already running
Cleanup Process use statements and make updateStatus() private
Skip Process environment tests on Travis CI
Bump dev-master branch alias in ChildProcess composer.json
Complete README for ChildProcess component